Persons travelling on a boat across a body of water are exposed to the possibility of falling overboard, and of drowning, in part because of confusion or an inability to determine the present location of the overboard person. If the boat from which the person falls overboard is a small keel boat, the boat master may be able to tum the craft around within a radius of 3-10 boat lengths after first becoming aware of the mishap; in a large keel boat, the distance required to turn is much larger because of the greater inertia of the large keel boat. The upper surface of a large body of water, such as a fiver, lake, sea or ocean, is seldom flat and often manifests a sequence of peaks and troughs ("swells") that serve to hide the exposed or above-water portion of the overboard person in the water. A large body of water is often perturbed by surface current that can carry a person or object in the water in an arbitrary direction at a rate greater than 3 feet per second. In such an instance, knowledge of the point of entry into the water of the overboard person is of little help in locating the person five minutes after the overboard incident. Thus, it is often crucial to determine the present location of the overboard person so that a boat or other watercraft can quickly return to that location and search for and retrieve the overboard person. Several workers in this field have developed approaches to (1) advise the boat's occupants of the overboard incident or (2) indicate the location where the overboard incident occurred or (3) assist in retrieval of the overboard person, once that person is located.
Sunken vessel locator apparatus that operates only if the vessel is submerged is disclosed in U.S. Pat. No. 3,686,656, issued to Richards. The apparatus includes a water-responsive switch and alarm with a water-tight battery is attached to the vessel and generates a distinctive sound underwater when the switch senses that it has become submerged. The distinctive sound might be used to determine the direction from which, but not the location of, the alarm that produces the sound.
Lieb discloses an alarm system, including a portable transmitter attached to the body of a person to be monitored, that is activated upon its immersion in water, in U.S. Pat. No. 3,810,146. The transmitter issues a distinctive signal that is sensed by a nearby receiver that activates an alarm to indicate that the monitored person (for example, a non-swimmer) is immersed in water. The transmitter switch is activated by electrical bridging of two electrodes through immersion of the volume between the electrodes in water or another liquid with relatively high electrical conductivity. The alarm system does not indicate the location of the monitored person in the water.
An alarm system that operates in a similar manner to that of Lieb is disclosed by Antenore in U.S. Pat. No. 4,079,364. The alarm system is automatically released from the wearer's body upon immersion in water and floats on the top surface of the water. The alarm is received above the water.
A Man Overboard Package, allegedly arranged to provide lifesaving apparatus for a person who has fallen overboard into water from a boat, is disclosed in U.S. Pat. No. 3,886,612, issued to Schnirel et al. The Package includes a rocket and rocket launcher that carries the remainder of the apparatus (inflatable life rings, etc.) to a position adjacent to the overboard person in the water. The Package provides no means of locating the overboard person, except for a line of length at most 600 feet that is dropped between the boat and the overboard person by the rocket and launcher.
U.S. Pat. No. 4,305,143, issued to Simms et al, discloses a Man Overboard sensor and rescue system. Part of the apparatus is worn on a person's clothing and is activated by immersion in water. This part of the apparatus, when activated, emits ultrasonic waves that are carried underwater to and sensed by a receiver carried on a boat. This receiver than activates an audible alarm or visual display, indicating that a person has fallen overboard, and may also cause flotation equipment and a marker buoy to be discharged from the boat. The marker buoy may or may not be adjacent to the person overboard, and the buoy's location would have to be found by line of sight visual searching.
Man Overboard rescue apparatus for use on a sailboat is disclosed by McDonald in U.S. Pat. No. 4,343,056. After a person has fallen overboard and been located, a lifeline and attached life ring is tossed to that person, and the person is pulled to the boat by a boom structure attached to a mast on the sailboat. A similar invention, disclosed in U.S. Pat. No. 4,599,073 by Fryer et al, uses a flotation sling to lift the overboard person into the boat. Beckly, in U.S. Pat. No. 4,599,074, discloses a Man Overboard retrieval device including a boom attached to the bow or other exposed portion of a boat, to be used to reel an overboard person into the boat. U.S. Pat. No. 4,747,797, issued to Hindle, discloses Man Overboard retrieval apparatus including an inflatable ring or small boat, part of which converts to a sling to haul the overboard person out of the water and into the boat. Wilson et al, in U.S. Pat. No. 4,843,994, disclose use of a specially configured boat fender as an auxiliary float, to be thrown to a person who has fallen overboard from a boat. None of these apparati provides a means for locating the overboard person so that a boat can move to that location.
Otaka discloses a system for automatically stopping a boat when the sole crewperson on the boat falls overboard, in U.S. Pat. No. 4,630,205. The (sole) crewperson wears a radio transmitter that transmits a signal continuously at a certain intensity when that person is aboard the boat. The transmitted signal is attenuated or stops altogether when the crewperson falls overboard, and the resulting absence of signal causes the boat to stop. In another embodiment, the on-board system computes the accumulated time since the crewperson fell overboard and causes the boat to maneuver to return to the approximate location where this overboard incident occurred. The radio transmitter worn by the crewperson can transmit a rescue signal when the overboard incident occurs, but this signal contains no information on the location of the crewperson or the transmitter.
A liquid immersion switch that is activated when the switch wearer falls overboard from a boat is disclosed by Boe in U.S. Pat. No. 4,714,914. An alarm delay circuit is built in so that momentary or transitory immersion of the switch in water or another liquid will not activate the switch. The alarm signal carries no information on the location of the person overboard.
Rowland et al, in U.S. Pat. No. 4,813,025, discloses a person location and water safety system in which each person on board a boat wears a transceiver. A central station interrogates each transceiver in a fixed sequence, using both radio waves and ultrasonic waves, and each transceiver responds by a signal that is distinctive for that transceiver. The "round trip time" for the interrogation signal and response signal indicates the distance of the person wearing that transceiver from the central station. If the wearer falls overboard, the transceiver responds with a radio wave or an ultrasonic wave, according as the transceiver is above the water or submerged in water; and the central station senses and distinguishes between arrival of these two kinds of response signals. The central station uses an angie discrimination antenna to determine the direction from which the response signal arrives and uses the signal round trip time to determine approximately the location of the transceiver, and thus of the wearer. This system appears to require line of sight communication, above the water line or below the water line, between the central station and an individual transceiver. The Rowland et al patent also discusses several earlier patents that disclose water immersion alarm systems for boating safety, none of which provides adequate information on location of the alarm or the wearer in the water.
A portable military target locator using a Global Positioning System (GPS) is disclosed by Ruszkowski in U.S. Pat. No. 4,949,089. A rifle-laser rangefinder located at a known GPS position on the ground is used to determine the present location of a moving target, such as a tank on the ground, relative to the rangefinder. This information, together GPS-determined information on the rangefinder's location, is transmitted to another vehicle, such as an aircraft on a search-and-destroy mission. The rangefinder apparatus must remain at a known, fixed position and relies upon line-of-sight contact with the target.
U.S. Pat. No. 5,006,831, issued to de Solminihac, discloses positioning of an acoustic signal transmitter on the underside of a boat, for use in a Man Overboard situation. A boat occupant wears an alarm pack including an acoustic sensor. If the boat occupant falls overboard, the alarm pack receives the acoustic signal and causes, by remote control, the boat to jettison a beacon with its own signal into the water, to mark the approximate position of the person who has fallen overboard. The beacon may enter and reside in the water at some distance from the person in the water, and the beacon provides only line of sight visual information on this person's approximate location.
Stetson, in "When a crewmember falls overboard", Sail, Feb. 1993, pp. 48-54, discusses how a boat should attempt to return to the location where a crewmember has just fallen overboard, using a jib or tacking and gybing to execute a sequence of 12 steps to bring .the boat back to that location. Although a small keel boat may be able to turn and return in 3-10 boat lengths, a big keel boat typically will require much more room, and the line of sight to the overboard crewmember may be lost for some time interval. A method, other than line-of-sight contact, for keeping track of the overboard crewmember's present location in the water is not discussed.
What is needed is an approach: (1) that can instantly advise the boat's occupants that an overboard incident has occurred; (2) that can continually advise the boat's occupants of the approximate present location of the overboard person; (3) that does not require line-of-sight contact and constant vigilance in order to maintain such contact; (4) that does not require expenditure of great amounts of resources, such as electrical power, to obtain information on the present location of the person overboard; and (5) that is relatively compact and can be easily carried by a boat occupant or on a boat; (6) that can be used for measuring or indicating the location and/or rate of movement of a boundary between two dissimilar liquids, such as oil and water, on the surface of a body of water; (7) that can be used for real time studies of ocean currents, tides, or movement of other bodies of water, such as rivers; (8) that can be used to mark the present location of fishing nets that are set out on an ocean or sea and returned to at a later time; and (9) that can be extended to search-and-retrieval for overboard persons or objects using aircraft or other vessels.